The substrate removal efficiencies of the conventional (taper angle of 0 degrees) and three tapered anaerobic fluidized-bed bioreactors (taper angles of 2.5 degrees, 5 degrees and 10 degrees) predicted from a kinetic model correlated with the average biofilm thickness measured. The predicted substrate removal efficiencies of the three tapered bioreactors were higher than that of the conventional bioreactor. This result was also confirmed by the experimental study, implying the performance of the conventional bioreactor can be significantly enhanced by changing the geometrical configuration to a tapered form. Moreover, the rate-limiting step of acetate methanogenesis in the bioreactors was reaction-controlled and the biofilm was fully-penetrated.